Rule of 72 Calculator

The Rule of 72 estimates how many years it takes an investment to double: divide 72 by the annual percentage return. At 8% a year, 72 ÷ 8 = 9 years to double. It's a mental-math shortcut that's remarkably close to the exact compound-growth answer for the rates most investors see.

Very close for typical returns of about 6–10%, usually within a tenth of a year of the exact figure. It drifts a little at extremes — for very high rates the true doubling time is slightly longer than 72 suggests. For quick estimates it's more than good enough; this calculator shows the exact number beside it.

The exact doubling time is ln(2) ÷ ln(1 + rate), and ln(2) ≈ 0.693, so multiplying by 100 gives about 69.3. But 72 divides cleanly by 2, 3, 4, 6, 8, 9, and 12, making the mental math easy — a small accuracy trade for a big convenience. Some people use 70 for continuous compounding.

Yes — the same math applies to anything compounding. At 6% inflation, prices double in about 12 years (72 ÷ 6), halving your money's purchasing power. On a 24% credit card, the balance can double in about 3 years if you don't pay it down. Compounding cuts both ways.